Conventionally, endothermic reaction apparatuses having a double-tube type construction have been used to modify a hydrocarbon, through reaction, to form a hydrogen-rich gas. Such a double-tube type apparatus, as disclosed in U.S. Pat. No. 3,144,312, has the following advantages: (1) The overall length of a reaction tube can be reduced since the endothermic reaction catalyst layer placed in the reaction tube is of a double, or folded, construction; (2) an improved efficiency is achieved because a combustion gas and the reaction gas can be caused to flow in opposite directions and because the temperature of the raw feed gas introduced into the catalyst layer can be increased through heat exchange between it and the heated reaction gas exiting the catalyst layer; and (3) the apparatus can be of a compact construction since a burner can be mounted inwardly of an inner tubular wall of the reaction tube.
In such a double-tube structure, an inlet and an outlet of the endothermic reaction catalyst layer through which the raw gas flows are disposed adjacent to each other and spaced apart by a cylindrical partition wall interposed therebetween. During the reaction, the temperature at the inlet of the catalyst layer is different from the temperature at the outlet, so that the heat is transferred from the side of a higher temperature to the side of a lower temperature through the partition tubular wall. This is disadvantageous in that the reaction fails to proceed smoothly. For example, when the properties of methane gas are to be modified by steam, the temperature at the inlet of the endothermic reaction catalyst layer is between 400.degree. C. and 550.degree. C. while the temperature at the outlet is between 650.degree. C. and 800.degree. C. Thus, a temperature difference of about 200.degree. C. always exists between the inlet and the outlet, so that the heat is always transferred from the outlet to the inlet through the cylindrical partition wall. As a result, the amount of heat at the outlet of the catalyst layer becomes inadequate, so that the temperature of the catalyst layer can not be increased to a sufficient level. This is disadvantageous in that the reaction gas which has undergone insufficient reaction flows out of the catalyst layer.